LARS - Laser Absolute Reference Spectrograph

Project and instrument

LARS, the Laser Absolute Reference Spectrograph of the Kiepenheuer-Institute is a scientific instrument for the ultramodern observation of the Sun at the Vacuum Tower Telescope (VTT) at the Observatorio del Teide on Tenerife. LARS enables the measurement of the solar spectrum of a selected field-of-view with the high-resolution Echelle-spectrograph of the VTT. Additionally, the emission spectrum of the newly installed Laser Frequency Comb is superimposed with the solar spectrum. Since each emission peak of the comb spectrum represents one well-defined frequency, the solar spectrum can be calibrated on an absolute wavelength scale. Figuratively, the comb serves as a ruler for the spectral lines. The accuracy is of the order of m/s or below and is therefore a multiple better than former devices. After the invention of the Laser Frequency Comb, awarded in 2005 with the Nobel Prize in Physics, the first astrophysical Frequency Comb was realized in 2008 at the VTT. The successful upgrade of the system in May 2016 by MenloSystems enables the continuous operation of this worldwide unique spectroscopic observation of the Sun.

Schematic setup of LARS. The emission spectrum of the pulsed Laser is mode-filtered and broadened with a Photonic Crystal Fiber (PCF) to the visible spectral range. By a fiber switch, the light from the Frequency Comb, the sunlight from the VTT, and other light sources can be switched and guided by a single-mode fiber to the Echelle spectrograph (after H.-P. Doerr).

Optical setup

The basis of the Laser Frequency Comb of LARS is a near-infrared Laser at 1060nm which sends out fs-pulses which generates a stable well-defined emission spectrum. Since the frequency spectrum resembles the look of a comb one speaks of a Frequency Comb. To guarantee the unambiguous identification of the modes, the comb spectrum with its repetition frequency of exactly 250MHz is filtered by Fabry-Pérot interferometers to a final mode separation of 8GHz. To enable the use of the Frequency Comb in a broad range, the spectrum is broadened by a Photonic Crystal Fiber (PCF) to the optical and near-infrared spectral range (480nm to 1500nm).

The light of the Frequency Comb can therefore be used for the absolute wavelength calibration of e.g. the solar spectrum. By single-mode fibers, the comb spectrum (480–750nm) is guided to the Echelle Spectrograph of the VTT. When the solar spectrum of an integrated field-of-view (1", 3", or 10") is fed to the spectrograph in a sequence with the comb spectrum, the solar spectrum can be calibrated on an absolute scale. In addition, further artificial light sources (e.g., Laser or Hollow Cathode Lamps) can be measured. The combination of the ultra-precise Frequency Comb and the high-resolution Spectrograph of the VTT provides the best conditions.

The sunspot was observed with the LARS Context Imager in the G-band spectrum of the Sun. For spectral observations there are three different field-of-views (1", 3", or 10") at choice.

Observations

LARS provides the scientific community with the possibility to measure the solar spectrum on an absolute wavelength scale. This allows the calibration of absolute Doppler velocities in the solar atmosphere. The accuracy of m/s and the high spectral resolution over the integrated field-of-view can serve as the perfect reference for e.g. spectro-polarimetric co-observations with other telescopes and instruments.